In recent years, with increasing concern for the environment, methods and systems have been devised to reduce the amount of refuse which must be deposited in landfills or dumped into the oceans.
Municipal refuse and garbage, for example, is increasingly processed to recover valuable components therefrom with the remainder being burned or otherwise treated so that only a reduced volume of waste need be permanently disposed of as ash or as sanitary fill. Waste and refuse processing plants recover a metallic component from the unburnt waste, this product being referred to as unburnt refuse scrap. In other systems, the refuse or garbage may be incinerated and the burnt product subjected to separation of metals, thereby yielding burnt refuse scrap.
Since the metal separation in the case of the processing of incinerator products is effected by magnetic means, the burnt refuse scrap can contain high proportions of iron.
Consequently, the term "refuse scrap" or terms of similar significance are used herein to refer to the product having a high content in metals and generally a comparatively high proportion of ferrous metal, obtained by the processing of garbage and refuse and especially municipal garbage and refuse. The distinction between an incinerator product and a product obtained without incinerator processing can be discerned in the distinctions made between burnt and unburnt refuse scrap below.
Burnt and unburnt refuse scrap contain a significant proportion of nonmetallic components. As noted, the separation of iron from the nonferrous components in a refuse generally is effected in garbage and waste processing installations by magnetic separation techniques. The iron-containing scrap which thus results and of which more than 90% is obtained from the burnt refuse scrap, i.e. the incinerator metal scrap, has hitherto been utilized as an additive to blast furnaces. However problems have been encountered with this method of disposing or utilizing the metal scrap.
For example, by comparison with other iron sources in the form of other scraps, the refuse scrap has a low metal content (about 60 to 70%) and a proportionately higher amount of nonmetallic components. Such metal content is between 20 and 30% less than that of other scrap sources for metallurgical processes.
The refuse scrap in the past has also had a low apparent specific gravity (piled weight or bulk density) generally ranging between 0.3 to 0.4 metric ton per m.sup.3. This means that large volumes of material have to be handled at high cost.
Finally, the chemical composition or analysis of the refuse scrap, especially with respect to sulfur (up to 0.1% by weight), tin (up to about 0.6% by weight) and copper, chromium and nickel, complicated metallurgical use of the refuse scrap in the aforedescribed manner.
Another disadvantage of refuse scrap, apart from its comparatively low metal content, is its comparatively high content of slag formers of slagging components. These accompanying slag formers increase the amount of slag which is produced and which thus must be handled and also are detrimental to the metallurgical process since the slag formers are overproportional by comparison to the metal added to such processes.
Indeed, the reduced basicity in terms of the ratio of calcium oxide to silicon dioxide (CaO/SiO.sub.2), of the slag materials entailed for incinerator refuse slag additional lime which had to be added in the steel-making process. If additional lime is not added, the lower final basicity of the slag results in a poor sulfur distribution between the metal bath and the slag so that the steel which results may have an excessive sulfur content.
Furthermore, it has been found that the unknown and variable iron oxide content of the refuse scrap can pose a significant problem. For example, it is difficult to establish the required steel tapping temperature if the amount of iron oxide introduced with the scrap is unknown thereby leading to excessive numbers of overcooled melts which, for effective casting must be afterblown in extra process steps at increased cost.
Indeed, the output of high-grade steel falls when afterblowing of a melt is required and significant amounts of iron can be lost because of entrainment of the iron into the slag as a result of afterblowing.
In part because of the iron oxide introduced and in part of the iron lost in the slag, added quantities of deoxidizing materials may be required at high cost and finally the larger quantities of deoxidizers which may be required tend to lead to steel of reduced purity.